1. Field of the Invention
The present invention relates to the structure of intake ports of a direct injection diesel engine.
2. Description of the Related Art
Recently, fuel injection systems of diesel engines have been improved, and thus fuel injection under high pressure has become possible. Therefore, some diesel engines are designed in such a manner that gas taken in each cylinder forms a swirl flow of a reduced degree to thereby improve volumetric efficiency, whereby combustion is realized with the minimum swirl flow. For example, Japanese Patent Application Laid-Open (kokai) No. H07-217437 discloses a technique in which intake ports are disposed in a twisted manner along the cylinder shape, to thereby decrease the degree of swirls and improve volumetric efficiency.
However, in the technique described in the above-described Japanese Patent Publication, since two intake ports of each cylinder are disposed in a twisted manner, the structure of a valve drive mechanism becomes complicated.
In order to solve the above-mentioned problem, there has been proposed a structure in which two intake openings of each cylinder of the engine are disposed parallel to a direction of row of the cylinders (see, for example, German Patent Publication No. DE19942169). In that structure, intake ports communicating with the intake openings are formed to have a straight shape, and are directed to extend along a tangential direction of the corresponding cylinder. Therefore, the valve drive mechanism is prevented from becoming complicated, and the degree of swirls is decreased in order to realize efficient combustion by means of the improved volumetric efficiency.
However, in the technique disclosed in the German Patent Publication, since the intake ports are formed to extend along a tangential direction of the corresponding cylinder, there has arisen a problem that intake gas having flown from each intake port into the combustion chamber decreases in flow velocity in the vicinity of the cylinder wall surface, thereby lowering volumetric efficiency. Moreover, since the intake gas passes through the intake opening and flows into the combustion chamber at a position away from the center of the cylinder (in the vicinity of the cylinder wall surface), the technique has a problem in that the rotation moment of the intake gas increases, and thus the degree of swirls becomes relatively high.